Neural organization and plasticity of language.

Powerful advances in neuroimaging techniques have added to and refined classical descriptions of the neurobiology of language in adults. Recent studies have employed these methodologies to study the nature and extent of plasticity of language-relevant aspects of cerebral organization in adults, in early and late bilinguals and in people who have acquired language through different modalities. Studies of children have documented dynamic shifts in cerebral organization over the course of language acquisition. Each of these different approaches has revealed constraints on the identity of the neural systems that mediate language; these studies have also described the marked and specific effects of language experience on the organization of these systems.

Event-related potentials during auditory language processing in congenitally blind and sighted people.

While behavioral studies have documented delayed language acquisition in blind children, other studies have revealed better speech discrimination abilities for blind than sighted adults. Several brain imaging studies have provided evidence for cortical reorganization due to visual deprivation but the cerebral organization of language in blind humans is not known yet. We hypothesized that the increasing specialization of language systems normally observed during development may not take place to the same degree in blind individuals since posterior visual areas do not receive their adequate input. On the other hand, we hypothesized that blind people, due to their greater reliance upon the auditory language signal, may process speech faster than sighted people. To test these assumptions, event-related potentials were recorded while 11 congenitally blind and 11 sighted adults matched in age, gender, handedness and education were engaged in a language task. Participants listened to sentences in order to decide after each sentence if it was meaningful or not. Incongruous sentence-final words elicited an N400 effect in both groups. The N400 effect had a left-lateralized fronto-central scalp distribution in the sighted but a symmetric and broad topography in the blind. Furthermore, the N400 effect started earlier in the blind than in the sighted. Closed class compared to open class sentence middle words elicited a more pronounced late negativity in the blind than in the sighted. These results suggest that blind people process auditory language stimuli faster than sighted people and that some language functions may be reorganized in the blind.

Auditory memory in congenitally blind adults: a behavioral-electrophysiological investigation.

Blind people must rely more than sighted people on auditory input in order to acquire information about the world. The present study was designed to test the hypothesis that blind people have better memory than sighted individuals for auditory verbal material and specifically to determine whether memory encoding and/or retrieval are improved in blind adults. An incidental memory paradigm was employed in which 11 congenitally blind people and 11 matched sighted controls first listened to 80 sentences which ended either with a semantically appropriate or inappropriate word. Immediately following, the recognition phase occurred, in which all sentence terminal words were presented again randomly intermixed with the same number of new words. Participants indicated whether or not they had heard the word in the initial study phase. Event-related brain potentials (ERPs) were recorded from 28 electrode positions during both the encoding and the retrieval phase. Blind participants' memory performance was superior to that of sighted controls. In addition, during the recognition phase, previously presented words elicited ERPs with larger positive amplitudes than new words, particularly over the right hemisphere. During the study phase, words that would subsequently be recognized elicited a more pronounced late positive potential than words that were not subsequently recognized. These effects were reliable in the congenitally blind participants but could only be obtained in the subgroup of sighted participants who had the highest memory performance. These results imply that blind people encode auditory verbal material more efficiently than matched sighted controls and that this in turn allows them to recognize these items with a higher probability.

Spatial attention to central and peripheral auditory stimuli as indexed by event-related potentials.

Young adult subjects attended selectively to brief noise bursts delivered in free-field via central and peripheral arrays of four loudspeakers each that were arranged along a semi-circle extending from the midline to 90 degrees right of center. Frequent "standard" stimuli (90%) and infrequent "target/deviant" stimuli (10%) of increased bandwidth were delivered at a fast rate in random order and equiprobably from all eight speakers. In separate runs, the subject's task was to selectively attend to the center or rightmost speaker, and to press a button to the infrequent "target" stimuli occurring at the designated (spatial) location. Behavioral detection rates and concurrently recorded event-related potentials (ERPs) indicated that auditory attention was deployed as a finely tuned gradient around the attended source. The attentional gradients were steeper for the central than the peripheral array, indicating that attention can be more sharply focused upon sound sources directly in front of the listener. The ERP data suggested that selection for location is accomplished in two distinct stages, with an initial broadly tuned filtering, followed by a more narrowly focused selection of attended-location deviants.

Hemispheric specialization for English and ASL: left invariance-right variability.

Functional magnetic resonance imaging (fMRI) was used to compare the cerebral organization during sentence processing in English and in American sign language (ASL). Classical language areas within the left hemisphere were recruited by both English in native speakers and ASL in native signers. This suggests a bias of the left hemisphere to process natural languages independently of the modality through which language is perceived. Furthermore, in contrast to English, ASL strongly recruited right hemisphere structures. This was true irrespective of whether the native signers were deaf or hearing. Thus, the specific processing requirements of the language also in part determine the organization of the language systems of the brain.

Attention to central and peripheral visual space in a movement detection task: an event-related potential and behavioral study. I. Normal hearing adults.

The effects of focussed attention to peripherally and centrally located visual stimuli were compared via an analysis of event-related brain potentials (ERPs) while subjects detected the direction of motion of a white square in a specified location. While attention to both peripheral and foveal stimuli produced enhancements of the early ERP components, the distribution over the scalp of the attention-related changes varied according to stimulus location. The attention-related increase in the amplitude of the N1 wave (157 ms) to the peripheral stimuli was greater over the parietal region of the hemisphere contralateral to the attended visual field. By contrast, the largest effects of foveally directed attention occurred over the occipital regions where the increase was bilaterally symmetrical. Additionally, the effects of attention on the ERPs were significantly larger for moving than for stationary stimuli, and this effect was greater for peripheral than for central attention. A long-latency positive displacement component (300-600 ms) was larger over the right than the left hemisphere during attention to the lateral visual fields, but was symmetrical in amplitude when central stimuli were attended. These results suggest that different pathways are modulated when attention is deployed to different regions of the visual fields. Further, they suggest that the special role of the right hemisphere in spatial attention may be limited to analysis of information in the visual periphery.

Attention to central and peripheral visual space in a movement detection task: an event-related potential and behavioral study. II. Congenitally deaf adults.

We compared the effects of focussed attention upon event-related brain potentials (ERPs) to peripherally and centrally located visual stimuli in congenitally deaf subjects (Ss). The results were compared with those obtained from a group of normal hearing Ss in the same paradigm. ERPs from deaf and hearing Ss displayed similar attention-related changes with attention to the centrally located stimuli. These included enhanced amplitudes of the N1 component (157 ms) over the occipital regions of both hemispheres. By contrast, with attention to peripheral visual stimuli, ERPs from deaf Ss displayed attention-related increases that were several times larger than those from hearing Ss and different in scalp distribution. Whereas for hearing Ss the principal effects of attention to peripheral stimuli occurred over the contralateral parietal region, in deaf Ss the effects were also observed over the occipital regions of both hemispheres. In addition, lateral asymmetries in behavior and the ERPs indicated a greater role for the right hemisphere in this task in hearing Ss, but predominance of the left hemisphere in deaf Ss. These results suggest that auditory deprivation since birth has major effects on the development of the peripheral visual system. The specific pattern of group differences is discussed in relation to other studies of the effects of unimodal deprivation on the development of remaining modalities.

Attention to central and peripheral visual space in a movement detection task. III. Separate effects of auditory deprivation and acquisition of a visual language.

We employed event-related brain potentials (ERPs) and measures of signal detectability to compare attention to peripheral and central visual stimuli in normal hearing subjects who were born to deaf parents (HD Ss) and whose first language was American Sign Language (ASL). The results were compared with those obtained from normal hearing Ss and congenitally deaf Ss in the same paradigm. Task performance and ERPs during attention to the foveal region were similar in the 3 groups. In contrast, with attention to the peripheral stimuli the deaf Ss displayed attention effects over the occipital regions of both hemispheres that were several times larger than those in the hearing and the HD Ss. However, both HD and deaf Ss displayed lateral asymmetries in behavior and ERPs that were opposite in direction to those of the hearing Ss. Whereas hearing Ss detected the direction of target motion better when it occurred in the left visual field, deaf and HD Ss performed better for right visual field targets. Consistent with these results, the amplitude of the attention-related increases in the ERPs were larger from temporal and parietal regions of the right than the left hemisphere in hearing Ss, but were larger from the left than the right hemisphere in both the HD and the deaf Ss. These results suggest that auditory deprivation and the acquisition of a visual language have marked and different effects on the development of cortical specializations in humans.

Auditory event-related potentials in the squirrel monkey: parallels to human late wave responses.

Event-related potentials (ERPs) were recorded from the brain surface in squirrel monkeys during the presentation of two auditory stimulus paradigms which have previously been utilized to elicit scalp-recorded ERPs in humans. In the first paradigm, inter-stimulus interval (ISI) was systematically varied during the presentation of a series of tone pips. The tones produced a negative (70 ms)-positive (130 ms) sequence of components similar in morphology to the human scalp-recorded N1-P2 'vertex' potential. The amplitude of the N70 and P130 components recorded from midline electrodes decreased with decreasing ISI, as previously shown for the human vertex potential. However, this amplitude change with ISI was not observed in ERPs recorded from lateral frontal and temporal electrodes. These results agree with previous studies of monkeys and humans which suggest at least two different sources contribute to N1-P2 components recorded in response to tones. The effects of stimulus probability and novelty on ERP morphology and amplitude were studied in the second paradigm. ERPs elicited by frequent (P = 0.92) and infrequent (P = 0.08) tone pips presented in an unpredictable order were compared. N70 - P130 components were produced by both stimuli, and the infrequent stimuli also elicited a broad, long latency (300 ms) positive complex that decreased in amplitude with repeated presentations. In humans the same infrequent auditory stimuli produce a frontally distributed late positive component that has been interpreted as indicating the activation of orientation mechanisms or of a 'mismatch detector'. These data suggest that in these paradigms squirrel monkeys exhibit ERPs which are similar in several respects to ERPs recorded to identical stimuli in humans.

Altered visual-evoked potentials in congenitally deaf adults.

Visual-evoked potentials recorded from the scalp of congenitally deaf adults were significantly larger over both auditory and visual cortical areas than in normal hearing adults. Over temporal and frontal areas peripheral stimuli presented at long intervals elicited N150 components which were larger in deaf than in hearing subjects. Over occipital and parietal areas peripheral and foveal stimuli elicited larger P230 components in deaf than in hearing subjects. These results imply that early auditory experience influences the organization of the human brain for visual processing.

Effects of interstimulus interval on auditory event-related potentials in congenitally blind and normally sighted humans.

To test the hypothesis of auditory compensation after early visual deprivation, congenitally blind and sighted adults performed an auditory discrimination task. They had to detect a rare target tone among frequent standard tones. Stimuli were presented with different interstimulus intervals (ISIs) (200, 1000, 2000 ms) and the auditory-event related potentials to all tones and reaction times to targets were recorded. Increasing ISIs resulted in an increasing amplitude of the vertex response (N1-P2) in both groups, but this amplitude recovery was more pronounced in the blind. Furthermore, targets elicited larger and more posteriorly distributed N2 responses in the blind than in the sighted. Since target detection times were shorter in the blind as well, these findings imply compensatory adaptations within the auditory modality in humans blind from birth.

Electrophysiological studies of language and language impairment.

The organization of language-relevant brain systems was examined in normally developing and language-impaired children. Atypical patterns of brain activity were observed in subsets of children with specific language impairment (SLI) for both sensory (auditory and visual) and language processing. However, it was not the same groups of children who displayed abnormalities across the different tasks. The results supported a multiple-factors and multiple-subtypes framework for interpreting the neurobiology of SLI. The roots of SLI were also considered in normal infants and late talkers in studies of primary language acquisition. These studies suggest that the organization of neural systems important in language acquisition display dramatic changes during this time. Some of these are linked to the attainment of language milestones and appear to be independent of chronological age. Moreover, abnormalities in the lateral organization of electrophysiological activity may help predict which late talkers will catch up and who will later display SLI. More generally, the event-related potential technique is a powerful tool in studying the neurobiology of language and language impairment.

Language processing in men at risk for alcoholism: an event-related potential study.

Event-related potentials (ERPs) were recorded from a group of men with (FH+) and without (FH-) a family history of alcoholism. ERPs were recorded over the left and right cerebral hemispheres and over midline locations while subjects performed a letter rhyming task. The ERPs to the letters displayed major group differences in a negative component with a latency of 430 msec (N430). The amplitude of N430 was significantly smaller in the FH+ as compared to the FH- subjects. These group differences were more pronounced in ERPs recorded from over the right than the left hemisphere. In addition, in the FH+ group only the latency of N430 was linearly related to personal drinking habits. These results suggest that (1) certain brain functions associated with the language processing required in this task are different in men at high and low risk for the development of alcoholism and (2) moderate social use of ethanol may have more pronounced effects on language-relevant brain functions in FH+ than in FH- individuals.

Neural systems mediating American sign language: effects of sensory experience and age of acquisition.

ERPs were recorded from deaf and hearing native signers and from hearing subjects who acquired ASL late or not at all as they viewed ASL signs that formed sentences. The results were compared across these groups and with those from hearing subjects reading English sentences. The results suggest that there are constraints on the organization of the neural systems that mediate formal languages and that these are independent of the modality through which language is acquired. These include different specializations of anterior and posterior cortical regions in aspects of grammatical and semantic processing and a bias for the left hemisphere to mediate aspects of mnemonic functions in language. Additionally, the results suggest that the nature and timing of sensory and language experience significantly impact the development of the language systems of the brain. Effects of the early acquisition of ASL include an increased role for the right hemisphere and for parietal cortex and this occurs in both hearing and deaf native signers. An increased role of posterior temporal and occipital areas occurs in deaf native signers only and thus may be attributable to auditory deprivation.

Sensitive periods differentiate processing of open- and closed-class words: an ERP study of bilinguals.

The goal of this study was to test the hypothesis that neural processes for language are heterogeneous in their adaptations to maturation and experience. This study examined whether the neural processes for open- and closed-class words are differentially affected by delays in second-language immersion. In English, open-class words primarily convey referential meaning, whereas closed-class words are primarily related to grammatical information in sentence processing. Previous studies indicate that event-related brain potentials (ERPs) elicited by these word classes display nonidentical distributions and latencies, show different developmental time courses, and are differentially affected by early language experience in Deaf individuals. In this study, ERPs were recorded from 10 monolingual English speakers and 53 Chinese-English bilingual speakers who were grouped according to their age of immersion in English: 1-3, 4-6, 7-10, 11-13, and >15 years of age. Closed-class words elicited an N280 that was largest over left anterior electrode sites for all groups. However, the peak latency was later (>35 ms) in bilingual speakers immersed in English after 7 years of age. In contrast, the latencies and distributions of the N350 elicited by open-class words were similar in all groups. In addition, the N400, elicited by semantic anomalies (open-class words that violated semantic expectation), displayed increased peak latencies for only the later-learning bilingual speakers (>11 years). These results are consistent with the hypothesis that language subprocesses are differentially sensitive to the timing of second-language experience.

Lexical, syntactic, and stress-pattern cues for speech segmentation.

Many sources of segmentation information are available in speech. Previous research has shown that one or another segmentation cue is used by listeners under certain circumstances. However, it has also been shown that none of the cues are absolutely reliable. Therefore, it is likely that people use a combination of segmentation cues when listening to normal speech. This study addresses the issue of how young adults use multiple segmentation cues (lexical, syntactic, and stress-pattern) in combination to break up continuous speech. Evidence that people use more than one cue at a time was found. Furthermore, the results suggest that people can use segmentation cues flexibly such that remaining cues are relied upon more heavily when other information is missing.